In last year's News Letter (MGCNL 56:4-8, 1982), it was reported that Mu activity was lost as Mu stocks were inbred. One possible explanation of this loss could be the failure to transmit Mu as the dose of Mu increases. This might be accomplished by a Mu-induced abortion of ovules and pollen grains carrying high doses of Mu. That this does not take place in stocks that have been propagated by outbreeding, the usual method of propagation, is well established by the results of many such crosses. At least 87% of all outcross plants receive Mu. In last year's report we reported observations on ear sterility which might be indicative of ovule abortion in inbred populations. No consistent pattern of abortion was observed that would suggest high Mu doses result in such abortion.
This past summer we grew populations of Mu2 (Mu1 x Mu1), Mu4 (Mu2 x Mu2), Mu8 (Mu4 x Mu4), Mu16 (Mu8 x Mu8) and Mu32 (Mu16 x Mu16) and classified them for pollen abortion (Table 1).
Again, there is no consistent pattern that would indicate Mu-induced pollen abortion is taking place. It is possible that pollen grains with high doses of Mu might not appear abortive and yet not be able to function or compete against normal pollen grains (with few or no Mu's). However, in producing these Mu stocks, each generation the female parent theoretically contributes as many Mu's to the next generation as the male parent. Since there is no evidence of Mu-induced sterility on ears of such a cross, the female-contributed Mu's should be transmitted, and thus with inbreeding a certain minimal level of Mu, the female transmitted level, should be maintained. This has not been observed.
In sum, there is no good evidence that the loss in Mu activity when Mu stocks are inbred is due to selection against gametophytes with high Mu doses.
Table 1. Pollen sterility in inbred Mu stocks.
Donald S. Robertson
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